Power combiners or dividers are used in a wide variety of signal combination or distribution applications. By way of example, one such application is in radio frequency (RF) phased array receivers and transmitters. Generally, a power combiner combines multiple input signals into a single output signal, and a power divider divides a single input signal into multiple output signals. Power combiners or dividers that do not require any power supply are referred to as passive combiners or dividers. The same passive circuitry can be used as both a combiner and a divider.
FIG. 1 shows a simple combiner or divider comprising passive circuitry 100 and three ports denoted Port 1, Port 2 and Port 3. This device is configured to function as a combiner if Port 1 is an output and Ports 2 and 3 are inputs, and is configured to function as a divider if Port 1 is an input and Ports 2 and 3 are outputs. It should therefore be appreciated that any passive power combiner described herein can be represented as a passive power divider by simply considering the input ports of the combiner to be the output ports of the divider and the output port of the combiner to be the input port of the divider.
Desired properties in a passive combiner or divider include low insertion loss, compact circuit area, and isolation between the input ports for the combiner or the output ports for the divider. For simplicity, the following description will focus primarily on power combiners, with the understanding that extension to equivalent power dividers is straightforward.
FIG. 2 shows a conventional N-way Wilkinson power combiner. This combiner includes N input ports denoted Port 1 through Port N, and a single output port denoted Port N+1. At a particular frequency fo, an ideal Wilkinson combiner is lossless and the N input ports are isolated from one another. A quarter-wave transmission line of impedance √{square root over (N)}Z0 is coupled between each input port and the output port. An isolation resistor network includes isolation resistors NZ0 coupled between Ports 1 and 2, Ports 2 and 3, and so on. For additional details on this type of combiner or divider structure, see E. J. Wilkinson, “An N-way hybrid power divider,” IRE Trans. on Microwave Theory and Tech., pp. 116-118, January 1960.
While a Wilkinson combiner is well-suited for certain applications, a major challenge is the fact that the inputs have to be physically close to each other in order to connect the isolation resistor network to the inputs. There are many combiner applications, such as the above-noted phased array receiver, in which the input sources may be located relatively far from each other. Using a Wilkinson combiner in such applications will often require unduly long lines to connect the isolation resistor network to the inputs, thereby increasing both insertion loss and the required circuit area.
Similar problems arise in differential Wilkinson power combiners, such as the simple two-way combiner shown in FIG. 3. Like the single-ended inputs in the N-way combiner of FIG. 2, the differential inputs at Ports 2 and 3 in this two-way combiner have to be located close to each other in order to achieve the desired isolation.
FIG. 4 illustrates a conventional two-way Gysel power combiner. This combiner includes two input ports denoted Port 2 and Port 3, a single output port denoted Port 1, transmission lines L0 through L6, and resistors R1 and R2. There are no isolation resistors coupled between the inputs. Instead, this combiner uses the transmission lines L3, L4, L5 and L6, and the resistors R1 and R2, to achieve isolation. For additional details on this type of combiner or divider structure, see U. H. Gysel, “A New N-Way Power Divider/Combiner Suitable for High-Power Applications,” IEEE MTT-S IMS Digest, May 1975, pp. 116-118. While in this structure the inputs no longer need to be located close to each other, the lines L3, L4, L5 and L6 have a total length of λ, where λ is the wavelength at the frequency of interest. This requirement for long transmission lines between the input ports unduly increases the required circuit area.